Process for the decomposition of gases low in ethane, preferably for the production of pure ethylene



H. LINDE 3,091,095 PROCESS FOR THE DEcoMPoSITIoN 0F GASES Low 1N May 28, 1963 ETHANE, PREFERABLY FOR THE PRODUCTION OF PURE ETHYLENE 2 Sheets-Sheet 1 Filed Aug. 19. 1960 H. LINDE May 28, 1963 3,091,095 PROCESS FOR THR DECOMPOSITION OF GASES LOW IN ETHANE, PREFERABLY FOR THE PRODUCTION OF PURE ETHYLENE 2 Sheets-Sheet 2 Filed Aug. 19, 1960 precoolng soge ton SEPGQE s2 warm wafer 1N VENTOR HERMANN LINDE daf ATTORNEYS BY fm) United States Patent 3,091,095 PRCESS FR THE DECOMPOSTION 0F GASES LW EJ ETHANE, PREFERABLY FOR THE PRO- DUCTIN 0F PURE ETHYLENE Hermann Linde, Munich-Sullo, Germany, assigner t0 Gesellschaft fr Lindes Eismaschinen Aktiengesellschaft, Munich, Germany, a German company Filed Aug. 19, 1960, Ser. No. 50,760 Claims priority, application Germany Aug. 21, 1959 1 Claim. (Cl. 62-28) The working up of hydrocarbon gases low in ethane but possibly high in C3 hydrocarbons in ethylene systems encounters certain difficulties. In fact, due to the W ethane content there is usually a shortage of C2-contain ing reux liquid in the precooler to eliminate the C3 hydrocarbons Ias completely as possible. Such gas mixtures having a low ethane content as mainly those resulting from cracking oil and light gasoline, are prepared for the production of ethylene and propylene. The consequence of this shortage is that the C3 hydrocarbons are entrained into the low-temperature part of the separating plant, -and that they appear there in the sump of the ethylene column and collect in impermissibly high -amounts in the ethane fraction. Moreover, due to the elevated content of C3 hydrocarbons in the ethane fraction, the sump temperature of the ethylene-ethane separating column rises, which is extremely annoying for the operation of the system. This refers to concentrations of at least 4 or 5% by volume to about or 20% by volume of C3 hydrocarbons.

It is the object of the present invention to prevent entrainment of C3 hydrocarbons from the precooler into the lowtemperature part of the separating plant because of too low an ethane content in the crude gases. Therefore, in the separation of C3 hydrocarbons from the C1 and C2 hydrocarbons, eiiected by precooling, the yield of C3 hydrocarbons is to be increased in 4such cases Iand the ethane purity improved.

According to the process of the invention this object is achieved in that a portion `of the ethane returning as pure decomposition product from the system is introduced into the crude gas line ahead of the crude gas compressor. An optimum eiect is obtained when the portion of pure decomposition product recycled is such that in the stationary state the ethane concentration of the crude gas going Ito the decomposition apparatus is about 5 to 10% by volume.

The process of the invention will now be further explained, with reference to the accompanying drawing, in which:

FIG. 1 is a block diagram or flow sheet illustrating a procedure falling Within the scope of the present invention; and

FIG. 2 is a diagrammatic representation of apparatus operable for use in carrying out the process of the application, including a scrubbing step as described herein.

First, as shown in FIG. 1, the crude gas is compressed to about 30 atmospheres with removal of CO2 yand H28 (step l), and then is precooled to about 230 K. (step 2). This causes the C3, and higher, hydrocarbons as Well as the amounts of C1 and C2 hydrocarbons corresponding to the equilibrium conditions to separate out. The resulting precooling condensate enters a C2/ C3 fractionating column operated at atmospheres (step 5), from which the C3, and higher, hydrocarbons are withdrawn as sump liquid. The reux liquid of this column consists mainly of ethane. `If the amount thereof in the crude gas or in the precooling condensate is too small, then the C3 hydrocarbons cannot be washed out -of the head product of the C2/ C3 column completely enough and they pass together with the draining methane, ethane, and ethylene into the methane column (step 6), into which is fed also the bulk of CH4, C2H4, C2H3 and C2H2 separated from the C3-free Mice crude gas stream 'by deep cooling to 155 K. (step 4). After the separation of the methane, the C2 hydrocarbons ow ltogether with any C3 hydrocarbon impurities into the ethylene column (step 7), Where the ethane is separated as liquid. In the processing of gases low in ethane there concentrate here in the :sump of the ethylene column all C3 hydrocarbons which due to the shortage of C2-containing reflux liquid in the C2/ C3 column have been entrained into the low-temperature portion. Thereby the yield of C3 hydrocarbons is reduced, and, moreover, the purity of the resulting ethane is greatly imp-aired.

According :to the invention it is now proposed, in the decomposition of hydrocarbon gas mixtures low in ethane, to returnV a portion of the pure ethane recovered from the system to the crude gas compressor together with the entering crude gas (step 1). By this measure the ethane content of lthe crude gas supplied to the precooler is increased. In this way it is then possible to form highly concentrated C2-hydrocarbons-containing condensate at the head of the C2/C3 column, whereby Ithe Washing out of C3 hydrocarbons is promoted. For this reason, and also because now the ethane fraction drawn from the sump of the ethylene Icolumn is increased -by the recycling of the ethane, the C3-hydrocarbons-content in the sump of the ethylene column decreases. The resulting reduction `of the sump temperature again involves a considerable relief for the operation of the system.

The process of the invention may -be further explained by reference to FIG. 2 as basis.

The crude gas is compressed in compressor 1a, freed from H2S1and CO2 in the alkaline purification towers 1b, and then-after additional compression in the compressor output stage-is subjected .to precooling. For this purpose the compressed crude gas is led into heat-exchanger 2a in counterow to the cold separation products leaving the low temperature part of the plant, and subsequently is cooled in cooler 2b by the use of ammonia. This cooler 2b, which forms part of the precooling stage, acts as reflux condenser its function being the complete separa- -tion of the C3 and higher valence hydrocarbons from the ga-s mixture. The separation is Aaccomplished by condensation of the C3 and higher Valence hydrocarbons and by the simultaneous washing out of any C3 hydrocarbons which are still in gaseous state under vapor pressure at precooling temperature (230 K.) by means of a C2-containing uid. Therefore, the crude gas must contain a sufficiently large amount of C2 hydrocarbons to insure eicien-t operation of the installation.

The condensation product from heat-exchan ger 2 is then separated :from water in a separator 2c and thereupon is combined with Ithe condensation product from cooler 2b. This fluid is then led into, and rectified Within, the C2/ C3 tower 5a. The C3 (and higher valence) hydrocarbons collect in the sump of tower 5a at point 5b, and the C2 hydrocarbons are removed at -point 5c, at the top of the tower. This top product is dried in drying vessel 3' and from the product there is condensed, in the condenser, by use of vaporizing NH3, a reux which consists mainly of ethane. This reflux is fed into the tower 5a by Way of pipe 5e. The dry gaseous C2 mixture, when leaving the cooler 5d, and the gas mixture dried in drying vessel 3, when leaving the cooler 2b, are then conducted to the lowtemperature part of the plant Where they are separated into their components (see steps 4, 6, 7 -and 8 illustrated in FIG. 1).

-If |the crude gas has a comparatively low ethane content with respect to the content of C3 (and higher valence) hydrocarbons, the 4amount of C2 hydrocarbonespecially the amount condensed by the precooler 2bwill not suihce to completely Wash out the C3 component which remains in gaseous state after the condensation of the C3 (and higher valence) hydrocarbons at the precooling temperature: in this event, the disadvantages, ex-

ying separation for the purpose of obtaining a head, or

top, product which contains C2 hydrocarbons exclusively and a sump product which contains C3 (and higher valence) hydrocarbons exclusively. The result will be that the head product of -the C2/C3 tower, conductedto the low-temperature part of the plant, will also contain C3 hydrocarbons, thereby compounding the disadvantageous conditions described in the first paragraph.

The invention lavoids :these disadvantages by adding to, and mixing with the crude gas, a large amount of the ethane gained in the form of a separation product prior to the compression of the crude gas as shown in FIG. 2 -at point A; the ethane content of the crude gas will therefore amount to approximately 5 to 10 vol. percent.

The separation products withdrawn from the deep cooling stage (step 4), the methane column (step 6), the ethylene column (step 7) Iand `the acetone wash, are warmed lby indirect heat-exchange with the gases to be separted in the precooling stage (step 2) and deep cooling stage-(step 4) thereby cooling said gases `to be separated.

I claim:

In the process of obtaining substantially pure ethylene from crude hydrocarbon gas mixtures, containing ethylene and having a low ethane content Iand a C3 hydrocarbons content which is high relative to the ethane content, in la ldecomposition system in which the crude gaseous mixture is compressed lwhereby toseparate out any CO2 and H28 contained in thefcrude hydrocarbon gas mixture, precooled whereby to liquefy the C3 and higher hydrocarbons as well Ias amounts of C1 and C2 hydrocarbons corresponding to equilibrium conditions, and then the resulting condensate is fractionated using ka reuxing liquid consisting mainly of C2 hydrocarbons, the improvement which consists in increasing the yield of C3 hydrocarbons and simultaneously 4improving the purity of the ethane fraction 4by returning to the crude gaseous mixture, prior to compression thereof, such an amount of the substantially pure ethane fraction produced in the decomposition system thatl in the stationary state the ethane concentration of the gas mixture .supplied to the decomposition system is from about 5to about 10% Iby volume thereby increasing, `in the preeooling stage, the amount of C2 hydrocarbons-containing liquid necessary for completely liquefying by scrubbing the C3, and higher, hydrocarbons.

References Cited in the le of this patent UNITED STATES PATENTS 2,627,318 Swerdlo Feb. 3, 1953 2,731,810 Hachmuth Jan. 24, 1956 2,769,321 Stiles Nov. 6, 1956 2,804,488 Cobb Aug. 27, 1957 2,953,905 Chrones et al Sept. 27, 1960 

